BOSTON , Massachusetts -LRB- CNN -RRB- -- The operator of a trolley that rear-ended another trolley should have been able to see the other vehicle was stopped 480 feet ahead , a federal investigator said Monday .

Passengers walk past firefighters at a Green Line station in Boston after the trolley collision Friday evening .

The trolley driver has told investigators he was text messaging during Friday night 's collision , which injured 20 people .

The Massachusetts Bay Transportation Authority , which previously had banned operators from using cell phones and other portable devices , now has told employees to leave the devices at home while on duty , National Transportation Safety Board member Debbie Hersman said Monday .

Hersman also noted investigators have determined that the operator may have missed some crucial indications of a stopped trolley ahead on the tracks . Watch new transit rule on cell phones ''

`` There were several signals -- two green signals , a yellow signal and a red signal -- coming out of the station , and the point of collision occurred 80 feet past that red signal , '' Hersman said . `` We did a site-distance test , and we know that the operator had the ability to see the trolley stopped in front of him 480 feet in advance of the collision . ''

Boston officials said over the weekend that the trolley operator was using his cell phone . Numerous media outlets , including CNN 's Boston affiliates , have identified the driver as 24-year-old Aiden Quinn .

`` The operator of the striking train was interviewed at the hospital by two detectives , '' said MBTA general manager Daniel Grabauskas . `` He admitted that he was texting at the time of the accident . ''

The operator told detectives that , when he looked up , `` it was too late as he applied the brake and the train struck the other trolley , '' Grabauskas said .

He described himself as `` outraged . ''

`` We have reinforced for a number of years that the use of cell phones or any other kinds of electronic devices while operating a train or a bus is absolutely prohibited , '' Grabauskas said .

Though the investigation is ongoing , he said , the two-year employee will be fired if his version of events is confirmed .

None of the injuries was considered life-threatening , the MBTA said .

The collision happened at 7:18 p.m. ET Friday as the Green Line trains were traveling between the Park Street and Government Center stations in downtown Boston .

Both trains were traveling westbound when one train rear-ended the second , an MBTA official said .

A train operator also was text messaging last year in a California train crash that killed 25 people .

CNN 's Rob Frehse and Gary Bender contributed to this report .

@highlight

Federal investigator says Boston trolley crashed 80 feet past a red light

@highlight

Trolley driver told investigators he was texting during wreck that injured 20 people

@highlight

Drivers already banned from using phones ; authority now says keep them at home